194 research outputs found
Gravity and Non-gravity Modes in the VIRMOS-DESCART Weak Lensing Survey
We analyze the weak lensing data of the VIRMOS imaging survey using
projections (called E and B-modes) of the two independents observed correlation
functions. The E-mode contains all the lensing signal, while noise and
systematics contribute equally to the E and B modes provided that intrinsic
alignment is negligible. The mode separation allows a measurement of the signal
with a \sqrt{2} smaller error bars, and a separate channel to test for
systematic errors. We apply various transformations, including a spherical
harmonic space power spectrum C^E_l and C^B_l, which provides a direct
measurement of the projected dark matter distribution for 500<l<10^4.Comment: accepted version, minor changes, 18 pages including 6 figure
Sheer shear: weak lensing with one mode
3D data compression techniques can be used to determine the natural basis of
radial eigenmodes that encode the maximum amount of information in a
tomographic large-scale structure survey. We explore the potential of the
Karhunen-Lo\`eve decomposition in reducing the dimensionality of the data
vector for cosmic shear measurements, and apply it to the final data from the
\cfh survey. We find that practically all of the cosmological information can
be encoded in one single radial eigenmode, from which we are able to reproduce
compatible constraints with those found in the fiducial tomographic analysis
(done with 7 redshift bins) with a factor of ~30 fewer datapoints. This
simplifies the problem of computing the two-point function covariance matrix
from mock catalogues by the same factor, or by a factor of ~800 for an
analytical covariance. The resulting set of radial eigenfunctions is close to
ell-independent, and therefore they can be used as redshift-dependent galaxy
weights. This simplifies the application of the Karhunen-Lo\`eve decomposition
to real-space and Fourier-space data, and allows one to explore the effective
radial window function of the principal eigenmodes as well as the associated
shear maps in order to identify potential systematics. We also apply the method
to extended parameter spaces and verify that additional information may be
gained by including a second mode to break parameter degeneracies. The data and
analysis code are publicly available at
https://github.com/emiliobellini/kl_sample.Comment: 15 pages, 16 figures. Accepted version on OJ
Photometric redshifts: estimating their contamination and distribution using clustering information
We present a new technique to estimate the level of contamination between
photometric redshift bins. If the true angular cross-correlation between
redshift bins can be safely assumed to be zero, any measured cross-correlation
is a result of contamination between the bins. We present the theory for an
arbitrary number of redshift bins, and discuss in detail the case of two and
three bins which can be easily solved analytically. We use mock catalogues
constructed from the Millennium Simulation to test the method, showing that
artificial contamination can be successfully recovered with our method. We find
that degeneracies in the parameter space prohibit us from determining a unique
solution for the contamination, though constraints are made which can be
improved with larger data sets. We then apply the method to an observational
galaxy survey: the deep component of the Canada-France-Hawaii Telescope Legacy
Survey. We estimate the level of contamination between photometric redshift
bins and demonstrate our ability to reconstruct both the true redshift
distribution and the true average redshift of galaxies in each photometric bin.Comment: 14 pages, 12 figures, accepted for publication in MNRAS V2: Section
4.4 added. Significant additions to analysis in section 5.
Statistics of Dark Matter Halos from Gravitational Lensing
We present a new approach to measure the mass function of dark matter halos
and to discriminate models with differing values of Omega through weak
gravitational lensing. We measure the distribution of peaks from simulated
lensing surveys and show that the lensing signal due to dark matter halos can
be detected for a wide range of peak heights. Even when the signal-to-noise is
well below the limit for detection of individual halos, projected halo
statistics can be constrained for halo masses spanning galactic to cluster
halos. The use of peak statistics relies on an analytical model of the noise
due to the intrinsic ellipticities of source galaxies. The noise model has been
shown to accurately describe simulated data for a variety of input ellipticity
distributions. We show that the measured peak distribution has distinct
signatures of gravitational lensing, and its non-Gaussian shape can be used to
distinguish models with different values of Omega. The use of peak statistics
is complementary to the measurement of field statistics, such as the
ellipticity correlation function, and possibly not susceptible to the same
systematic errors.Comment: 5 pages, 4 figures, matches version accepted for ApJ
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